The invention relates to a boring head for a ground-boring device.
To produce boreholes in the field of horizontal boring, it is known to use “moles”, which have a reciprocating percussion piston controlled via a drive fluid. This piston transmits its kinetic energy when striking a contact surface of the housing or of the boring head and drives the ground-boring device step by step through the earth.
In this case, the boring head of the device fulfills two functions. Firstly, said boring head transmits the percussion energy of the percussion piston to the earth situated in front of it and loosens the latter. Furthermore, the loosened earth is to be laterally displaced, so that a borehole having the desired diameter is obtained. In addition, the boring head, due to special shaping, is to be provide for guidance and consequently for directional stability of the device.
To fulfill these tasks, various boring head forms have been developed.
Firstly, the prior art (for example DE 21 57 259 C) has disclosed boring heads having a conical parent body. Said boring heads are distinguished by the fact that the conical lateral surface, which is as smooth as possible, has minimum resistance to the direction of movement.
The conical boring heads partly have recesses in the direction of the longitudinal axis of the device. The loosened earth is transported outward through these recesses.
These boring heads have the disadvantage of low directional stability, so that the device is deflected from the desired path by lateral forces which are produced by striking an obstacle or by inhomogeneity within the earth.
In addition to the conical boring heads, “stepped heads” are known (for example from DE 101 12 985 A1). The stepped heads also have a diameter which increases from the tip but in a stepped manner. In longitudinal section, a boring head of this type therefore has two steps which meet at the tip of the head. By the lateral surface being divided into a multiplicity of sections, which run either (virtually) parallel to or perpendicularly to the longitudinal axis of the device, only small lateral forces occur at the boring head, so that stepped heads have good directional stability. A disadvantage is that high resistance to motion occurs in the advancing direction due to the stepped surfaces situated perpendicularly to the longitudinal axis. This results in a correspondingly reduced rate of advance.
Furthermore, it is known from DE 101 12 985 A1 to design the boring head or a plurality of elements of the boring head so as to be movable relative to one another in order to achieve more specific introduction of the percussion energy available.
Furthermore, DE 29 17 292 A1 discloses a rock-fragmenting tool for percussion machines, the boring head of this tool, in the front region, consisting of a number of cutters arranged in a star shape. The intermediate spaces between the cutters serve as discharge passages for excavated rock. Following behind the cutters of the boring head is a conical section of the boring device, this conical section deflecting the excavated rock in the direction of the borehole wall, where it is blown out through an annular gap between the borehole wall and the housing of the device.